1. Field of the Invention
The present invention relates to an electrophotographic printing device and more particularly the invention relates to an electrophotographic printing device, for the simultaneous printing of both sides of a recording medium.
2. Description of the Related Art
An electrophotographic printing device with which a recording medium can be printed on both sides is known from PCT Application WO-91/13386. The electrophotographic printing device described therein, in particular for printing single sheets, contains one or more printing modules in which the processing means for carrying out the electrophotographic process are grouped together. The printing modules are designed as separate assemblies for to be secured in a housing of the printing device and have fitting elements for fitting the printing modules next to one another in the housing. Each printing module contains an intermediate carrier with associated fixing elements for transferring a toner image generated on a photoconductor drum onto a recording medium and fixing it there. The printing device can be extended from a single-color simplex printer to a multi-color duplex printer by multiple arrangement of the printing modules.
Another electrophotographic printing device which has two identical printing modules disclosed in this design is described in European Patent EP-A2-0 399 794. The printing modules are secured in such a way that a conveyor channel for the recording medium opens between them, heating means being provided on guide rollers for the simultaneous fixing of the toner images generated on the photoconductors.
An additional method for printing both sides of a recording medium with an electrophotographic printing device is described in U.S. Pat. No. 3,694,073, in which a toner image is transferred onto a front side of the recording medium with the aid of a first photoconductor drum and a toner image is transferred onto the rear side with second photoconductor drum. The toner images are fixed over several stages. First, a radiator following the first photoconductor drum partially fixes the toner so that the recording medium does not heat up too much and consequently the transfer of the toner image at the second photoconductor drum is not hindered. A further partial fixing is performed by a further radiator following the second photoconductor drum. The actual overall fixing of the toner images is performed by a subsequent fixing station with amplified radiators arranged on both sides of the recording medium.
This multi-stage fixing is necessary because the toner image is made up of loose toner particles that can become detached from the recording medium between the photoconductors. Multiple applications of heat on the recording medium in the course of the printing process deforms and stresses the recording medium which is made of paper. This impairs the print quality and the paper handling. In addition, the multiple arrangement of radiators with a high power consumption requires a powerful power supply with physical dimensions. This restricts the flexible use of the printing device.
With object of present invention is to provide an electrophotographic printing device of simple design for the simultaneous printing of both sides of a recording medium, which is not prone to malfunctions and which ensures gentle handling of the recording medium with the lowest possible power consumption.
A further object of the present invention is to construct the printing device in such a way that it can be constructed using standard parts depending on the specific requirements of customers.
These and other objects of the present invention are accomplished by the electrophotographic printing device of the present invention.